Recently, there is an increasing interest in energy storage technology day by day. As the application field of energy storage technology has been extended to mobile phones, camcorders, laptop computers, and even electric cars, many efforts have been made on research and development of electrochemical devices. In this aspect, electrochemical devices gain the most attention, and among them, attention is focusing on development of secondary batteries that can be recharged. More recently, in developing such batteries, research and development of new electrode and battery designs is being carried out to improve the capacity density and specific energy.
Among currently available secondary batteries, lithium ion batteries developed in the early 1990's have drawn particular attention due to their advantages in terms of higher operating voltages and much higher energy densities than traditional batteries using an aqueous electrolyte solution, for example, Ni—MH, Ni—Cd, and lead-acid batteries. However, lithium ion batteries have safety issues, such as fires and explosions, caused by the use of organic electrolytes, and their disadvantage is complex manufacturing.
It is very important to assess and ensure the safety of batteries. The most important consideration is that batteries shall not cause damage to users in the event of malfunction, and for this purpose, fire and smoke emission in batteries is strictly restricted by safety standards for batteries. Accordingly, many solutions are proposed to solve the safety problem.
For example, lithium ion batteries and lithium ion polymer batteries generally have a polyolefin based separator to prevent a short circuit between a positive electrode and a negative electrode. However, the downside of the polyolefin based separator is that the separator returns to its original size by thermal contraction at high temperature due to the properties of the separator material, for example, the properties of polyolefins that generally melt at 200° C. or less and processing properties, for example, property to undergo a stretching process for adjusting the pore size and porosity. Accordingly, when battery temperature increases due to internal/external impacts, the separator contracts or melts and there is a high likelihood that a short circuit will occur between positive and negative electrodes, hence the batteries have a high risk of explosions caused by emission of electrical energy.
To solve the problem, a composite separator having an inorganic coating layer formed on a polyolefin based film was proposed, but the composite separator needs improvements in terms of bonding between the separator and the electrode and the thickness.
Recently, an electrode that is surface-coated with inorganic particles and acts as a separator is proposed. However, when a separator function is imparted by forming an inorganic coating layer on the electrode surface, shut down does not take place at high temperature, leading to thermal runaway.